The present invention relates to a nickel or nickel alloy electroplating bath used for electroplating a conductor partially masked with an organic high-molecular resist layer, for example, a printed circuit board partially masked with a solder resist layer, and to an electroplating process using the bath. In particular, the present invention concerns a nickel or nickel alloy electroplating bath capable of electroplating the above conductor without floating (peeling) of the above resist layer during electroplating.
Nickel or nickel alloy electroplating has been used for printed circuit boards. Specifically, an organic high-molecular resist film (which is often called a solder resist film in the field of printed circuit boards) is partially formed on a printed circuit board, and a portion of the board excluding the area masked with the resist film is subjected to nickel or nickel alloy electroplating.
The above electroplating has been performed using a Watts type nickel electroplating bath mainly containing nickel sulfate and nickel chloride. Such a Watts type nickel electroplating bath, however, has been poor in macrothrowing power, and therefore, it has been expected to develop a new electroplating bath enhanced in macrothrowing power.
The nickel or nickel alloy plating baths enhanced in macrothrowing power have been known, for example, from Japanese Patent Publication Nos. Hei 2-22158, Hei 2-22160, Hei 2-44911, Hei 3-19308, and Hei 3-19309. In each of these plating baths, the concentration of a plating metal such as nickel is made relatively low, and an electrical conductive salt such as a halide, sulfate or sulfamate of an alkali metal, alkali earth metal, or aluminum is incorporated at a high concentration. In particular, a sodium salt or potassium salt has been practically used as the above electrical conductive salt.
The above-described nickel or nickel alloy plating bath enhanced in macrothrowing power, however, has problems. Specifically, in the case where a printed circuit board masked at a specific region thereof with the above resist layer is subjected to electroplating using the nickel or nickel alloy plating bath containing a sodium salt or a potassium salt in a large amount, there possibly occurs a floating (or peeling) phenomenon of the resist layer, although the macrothrowing power is undoubtedly enhanced. The floating of the resist layer leads to problems in that a copper underlayer of the printed circuit board is exposed and the copper layer thus exposed is corroded, and that the floating resist layer can no longer keep the solder resisting function resulting in occurrence of a solder bridging phenomenon upon soldering. These problems leads to critical defects of the printed circuit board. The floating of the resist layer has another problem. Specifically, although the resist layer is formed at the area where plating should not be performed, there is a possibility that a plating film is formed at a portion where the resist layer is floated (peeled).
It is to be noted that the above-described Watts type plating bath does not exhibit the floating phenomenon of the resist layer; however, as described above, it is poor in macrothrowing power.
In view of the foregoing, it has been required to develop a nickel or nickel alloy electroplating bath capable of giving a high macrothrowing power without floating of a resist layer.